Summary: Members of the MDR and MRP families of proteins are expressed on T cells, and T cell function appears to depend upon these "drug pumps." We have found that inhibition of one member of this family blocks cytokine secretion in response to T cell receptor stimulation. Since allergic diseases are mediated by CD4 T cells and type 2 cytokines such as IL-4 and IL-13, modulation of T cell function by inhibiting relevant drug pumps such as MRP1 may be a novel approach towards treating allergic diseases.